This invention relates to a device for the manufacture of diffraction gratings, and more particularly to a device which combines laser beam and fiber optic technology with a flexible mechanical design to facilitate the production of high quality holographic diffraction gratings.
A diffraction grating can be used to cause an incident light beam to be deflected and dispersed according to the well known principles of diffraction and interference. These gratings have wide usage in modern scientific devices such as monochrometers, spectrometers and lasers, and, in addition, mass-produced grating replicas which are selected for their beauty and novelty are found in art, advertising, and displays.
The manufacture of diffraction gratings using the holographic exposure method is well known in the prior art. A photographically sensitive material known as photo-resist is coated on a substrate (the grating blank) and located in a position at which two coherent beams of light intersect to create a three dimensional array of light and dark regions known as interference fringes. After exposure and development using certain etching chemicals, the pattern of regularly spaced bars and furrows on the grating surface will be directly related to the shape and intensity of the exposing interference fringes.
The production of interference fringes requires two coherent beams of light which are usually produced as secondary beams by the separation of a primary beam of coherent light from a suitable source such as a laser. In the prior art this requires the use of a complex assemblage of lasers, spatial filters, beam splitters and mirrors precisely located and oriented so as to create beams of the proper size and quality. Generally, one sets up and aligns the optics for one particular type of grating, and because maintaining alignment of the various components is difficult, if any changes are required the apparatus must be largely rebuilt.
Problems of alignment and realignment which are incurred in the prior art are especially difficult when producing specialized gratings such as crossed-line gratings, dual frequency gratings and holographic optical elements. A crossed-line grating is one that has furrows along both orthogonal x and y directions, and it is produced by exposing the grating blank and then rotating it 90.degree. and exposing it again. Precise alignment is essential.
A dual frequency grating is produced by superimposing two different diffraction gratings of slightly different groove spacing, with one frequency being slightly compressed or expanded in relation to the other. The process is tedious, requiring two exposures at different beam angles without disturbing the spatial filtering, collimation or beam quality.
Holographic optical elements (HOE) often require the use of beams with other than plane wave fronts to obtain lensing effects. One such element has a groove pattern consisting of concentric circles, resulting from exposure by one plane wave and one spherical wave. When light strikes such a groove pattern it will be affected much as if it had struck a lens, but the HOE is only a few microns thick, and thus provides significant weight savings compared to a lens. Considerable care must be taken to properly align the optical components.
It is therefore a primary object of this invention to provide an improved apparatus for the manufacture of diffraction gratings.
In the accomplishment of the foregoing object, it is another important object of this invention to provide an apparatus which is compact, portable, and permits flexibility in alignment.
It is another important object of this invention to provide an apparatus which is convenient to use for the manufacture of specialized gratings such as crossed-line gratings, dual frequency gratings and holographic optical elements.
Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.